"We don't have a very good sense of why people develop autoimmune disorders," study author Emily Somers said in a university news release.

"A large number of cases are not explained by genetics," she added, "so we believe studying environmental factors will help us understand why autoimmunity happens and how we may be able to intervene to improve health outcomes. In our study, exposure to mercury stood out as the main risk factor for autoimmunity," Somers said.

Somers is an associate professor in the departments of internal medicine in the division of rheumatology, environmental health sciences, and obstetrics & gynaecology at the University of Michigan Medical and Public Health Schools in Ann Arbor.

Somers and her colleagues reported their findings in Environmental Health Perspectives.

Relatively high amounts of mercury

The research team pointed out that swordfish, king mackerel and tile fish are all known to contain relatively high amounts of mercury. Lower levels are also found in shrimp, canned light tuna and salmon.

This makes any mercury-immune disorder connection troubling for women of childbearing years, the researchers noted, given that the U.S. Food and Drug Administration and the Environmental Protection Agency have long said that consuming up to 12 ounces of seafood a week is safe for pregnant women.

To explore risk factors for autoimmune disorders, the study authors focused on government data that looked at women between the ages of 16 and 49 between 1999 and 2004.

The result: the higher the exposure to mercury, the higher the rate of proteins called "autoantibodies". Such proteins are generated when a faulty immune system can no longer distinguish healthy cells from harmful ones, and their presence is considered an indicator and/or precursor of autoimmune disease.

While the study found an association between mercury exposure and the possible development of autoimmune diseases, it did not prove that mercury causes the diseases.

"The presence of autoantibodies doesn't necessarily mean they will lead to an autoimmune disease," Somers stressed.

"However, we know that autoantibodies are significant predictors of future autoimmune disease, and may predate the symptoms and diagnosis of an autoimmune disease by years," she explained.

"For women of childbearing age, who are at particular risk of developing this type of disease, it may be especially important to keep track of seafood consumption," she added.

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